This invention relates generally to method and apparatus for encoding binary data used, for example, in the digital signal recording, and more particularly to such method and apparatus by which an improved sequence of encoded binary digits to be modulated in the NRZI modulation is obtained from the binary data.
The digital signal recording utilizing the binary data is made with variations in residual magnetization on a magnetic recording medium, variations in geometrical shape formed on a recording medium shaped in a disc and so on, and has the feature that a great deal of information can be recorded in high density and kept in good preservation. In order to produce the variations in residual magnetization, variations in geometrical shape and so on as mentioned above, it is required to generate a recording signal, for example, a recording current in the form of a rectangular pulse train having the waveform which varies in response to the binary data, and therefore it is achieved to encode the binary data with a predetermined coding system to produce encoded binary digits and then to modulate the encoded binary digits in a predetermined modulation to obtain the recording signal. Various modulation systems have been proposed for the modulation for such a purpose and the NRZI modulation is one of them. In brief, the NRZI modulation is such a modulation system as to invert the level of the recording signal at the binary digit of "1" and not to invert the level of the recording signal at the binary digit "0 " to provide with two different levels to the recording signal.
In the waveform of the recording signal obtained by the NRZI modulation or the like, a minimum one of intervals between successive two level inversions is called the minimum run length.multidot.Tmin and this exercises an important effect upon recording characteristics. That is, the inversions in magnetization or the variations in geometrical shape are made on the recorded medium in accordance with the level inversions in the recording signal and, in reproducing process, reproduced pulses are obtained in response to reading out of the inversions in magnetization or the variations in geometrical shape, and distortions in the waveform of the reproduced pulses are caused by the mutual interference between successive two of the reproduced pulses and such distortions increase when the interval between successive two level inversions in the waveform of the recording signal is short. The increased distortions in the waveform of the reproduced pulses introduce easily a mistake in conversion of the reproduced pulses to the binary digits. Accordingly, the minimum run length.multidot.Tmin is desired to be long.
While a maximum one of the intervals between successive two level inversions in the waveform of the recording signal is called the maximum run length.multidot.Tmax and this affects easiness in self-clocking in the reproducing process. That is, since the reproduced pulses are not obtained during a period corresponding to the maximum run length.multidot.Tmax, it is impossible to produce a clock signal directry from the reproduced pulses at the period corresponding to the maximum run length.multidot.Tmax and the generation of the clock signal is easy to be inaccurate. This results in difficulty in the self-clocking. Accordingly, the maximum run length.multidot.Tmax is desired to be relatively short.
Further, when the level of the waveform of the recording signal, which forms the rectangular pulse train taking a positive level and a negative level, is unbalanced toward one of the positive and negative levels during a certain relatively long period, in other words, in case that supposing a minimum pulse width of the positive rectangular pulse is represented with +1, a pulse width which is X times as long as the minimum pulse width of the positive rectangular pulse is represented with +X, a minimum pulse width of the negative rectangular pulse is represented with -1 and a pulse width which is X times as long as the minimum pulse width of the negative rectangular pulse is represented with -X, the accumulation of such representations during the certain relatively long period takes a certain positive or negative value, it is true that the waveform of the recording signal contains a DC component in its frequency spectrum. The waveform of the recording signal which contains the DC component such as mentioned above is undesirably distorted in transmission, because the recording signal and reproduced signal are generally transmitted through AC coupling elements. Furthermore, in such a case, even if it is intended to reproduce the same waveform as the original waveform of the recording signal in the reproducing process, it is impossible to recover the DC component which has been omitted at the AC coupling elements. Accordingly, the waveform of the recording signal is desired not to contain the DC component.
In case that the NRZI modulation is adopted, since the level inversions in the waveform of the recording signal are caused at every binary digit of "1" in the encoded binary digits, the minimum run length.multidot.Tmin, the maximum run length.multidot.Tmax and the matter whether the waveform of the recording signal contains the DC component or not are determined by the manner of appearance of the binary digit of "1" in the encoded binary digits. Various systems for obtaining encoded binary digits to be modulated in the NRZI modulation by means of encoding the binary data have been previously proposed. For example, such a system as to convert every three digits of the binary data to six encoded binary digits as a unit block of the whole encoded binary digits has been already known as a system in which the long minimum run length.multidot.Tmin can be obtained. In this system, the minimum run length.multidot.Tmin has the period corresponding to one cycle of binary digit and a half of the binary data. However, there is a disadvantage that the waveform of the recording signal formed by modulating in the NRZI modulation the encoded binary digits obtained in accordance with this system contains the DC component. On the other hand, another system which is so designed that the waveform of the recording signal formed by modulating in the NRZI modulation the encoded binary digits obtained thereby does not contains the DC component has been also proposed already. However, in this case, there is also a disadvantage that the minimum run length.multidot.Tmin can not be more than the period corresponding to one cycle of binary digit of the binary data.
As mentioned above, none of the previousely proposed systems satisfies both requirements that the long minimum run length is to be obtained and the waveform of the recording signal formed by the NRZI modulation is not to contain the DC component. Incidientally, with regard to the maximum run length.multidot.Tmax, even if it is relatively long, it is not serious problem, because it is possible to generate accurately the clock pulse by means of use of a phase locked loop (PLL) circuit or the like at a clock generating portion in a reproducing apparatus.